This is a two-year feasibility study for high-impact auditory research. The purpose is to evaluate basic properties of adaptive recalibration of the chronic "gain" (amplification of supra-threshold information) of the auditory system in normal- hearing subjects. The idea of an adaptive auditory gain mechanism has been widely cited in the Tinnitus Retraining Therapy (TRT) literature, but it remains a controversial concept that has gone virtually unstudied in a rigorous scientific manner. The experimental hypothesis of this research is that estimates of loudness discomfort level (LDL) provide a functional index of chronic auditory gain. Further, the chronic gain can be manipulated either upward or downward in a controlled way by prolonged reduction or enhancement in the level of the external background sound to which a listener is exposed. To evaluate this hypothesis, 32 healthy normal-hearing volunteers will randomly receive one of two chronic external sound treatments: (1) enhanced background sound produced by wearable bilateral noise sources or (2) diminished background sound produced by wearing sound-attenuating earplugs bilaterally. After two weeks on the randomized treatment (and subsequent to complete functional recovery from the initial treatment), sequential crossover to the other treatment will occur. The effects of each chronic (two-week) treatment will be evaluated for functional changes (in audibility thresholds and LDLs) in pre-treatment, treatment, and post-treatment measurements. Findings of (1) increased LDLs (consistent with reduced chronic auditory gain), after prolonged exposure to enhanced external low-level sound, and (2) decreased LDLs (consistent with increased chronic auditory gain), after chronic exposure to reduced external low- level sound, will provide compelling evidence in support of the existence of a plastic chronic auditory gain process. Such results would have enormous implications for stimulating future research in both basic auditory theory and clinical science.